I had the same problem. I went to a champion 4 core radiator. I had to do a lot of "massaging" to get a larger unit in there. It still ran hot. I went out and bought a wideband O2 sensor and put it in. I was running way lean at part throttle. It was around 18:1-20:1 AF mixture. I adjusted the carb and all of my issues went away.
